The main reasons for unsatisfactory trades is being impatience and the fear of missing out. I've come across both of them, I often found myself being impatient and selling too quickly, which lead into losing a decent profit in the long run, while I've also sold when the market was facing a crash. Sometimes, we're also affected by what others are doing, if for instance the forum is going nuts about Ethereum's crash (just an example), we'll be tempted to also sell because that's what everyone is doing.
I guess, most of us are guilty of those actions at one point in our trading life. And even if we have had experience the same, it doesn't mean, we will not make the same mistakes again, especially if we need the money. This is when we usually make poor decisions, because we are in a rush to sell our coins.
But if you are not in a hurry, you can strategize how to at least minimize losses. If you know you are holding a valuable coin and it dropped in the market, don't sell. Because if it has solid grounds, it will go up again. This is when most holders are vulnerable to make wrong decisions. For the fear that it will not go up and continue to decline, they will sell at a loss.
Mistakes are to be made, and we won't stop making them. I've seen the cliché being spread in the forum, that you've learned from experience and your mistakes, so you won't repeat them again. I only partly agree with that statement, trading is unpredictable, and also involves great risk. Thus, we're bound to make the same errors again at some point.
For instance, I regret selling Bitcoin in December, I could also say that two years from now, for selling a month ago, and so on.